feat(i18n): standardize English and Simplified Chinese UI strings
- Move hardcoded user-facing text from Compose screens and dialogs into string resources
- Add and refine English and zh-rCN translations for settings, theme, about, backup, TOC, and book info flows
- Keep internal logs, exceptions, parser text, and comments unchanged unless user-facing
- Preserve existing behavior while improving localization consistency
dat-bi
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